1. Field of the Invention
The present invention relates to a method of forming an electrode for a surface acoustic wave (SAW) device and, especially, to an inter digital transducer (IDT) electrode and a method of forming the same.
2. Description of the Related Art
FIG. 5 shows a conventional IDT of an SAW device. An IDT pattern is made by forming a conductive film 2 with Aluminum (Al) on a piezoelectric substrate 1 made of LiTaO3 and etching the film 2 with a photoresist mask.
However, when the Al film is used for the IDT, there has been a problem that the withstand voltage per unit area is not sufficient due to the migration.
It has been proposed that a small amount of another metal is added to Al or a layered structure shown in FIG. 6 is employed to increase the withstand voltage. However, as the demands for a small or high-frequency SAW device increases, it is more difficult to obtain the satisfactory withstand voltage because the line width and thickness of the electrode must be made smaller and smaller to satisfy the high-frequency use.
FIG. 6 shows a conventional IDT having three-layered structure. First, second, and third conductive films 3, 4, and 5 are formed on the substrate 1 in this order. Then, the conductive films 3, 4, and 5 are dry-etched to form an IDT with a photoresist pattern.
One cause of the deterioration in the withstand voltage result from the problem of film structure besides the problem of pattern design. When a film is made of Al, voids and hillocks are produced by a stress in the film when high electric power is applied to the film, thus causing the deterioration. That is, the strain produced by the SAW causes the stress in the electrode films and when the stress exceeds the critical limit, the Al atoms move in the grain boundary, thus producing the voids inside the electrode or the hillocks in the surface of the electrode. Especially, when the hillocks are produced along the sidewall of the IDT pattern, adjacent electrodes are broken by short-circuit. In the SAW device in FIG. 6, the hillocks can not be prevented from being produced along the sidewalls of the electrode.
In the above dry-etching, when the electrode includes, besides Al, a metal whose halogenide has a high vapor pressure, such as Ti, Pd, and Si, the etching is completely performed without any residue. On the other hand, when the electrode includes a metal whose halogenide has a low vapor pressure, such as Mg, the halogenide remains as residue, thus causing corrosion. Especially, if such a metal is included with high concentration, the etching itself does not proceed. If over-etching is attempted to solve the incomplete etching, the substrate is also etched and adverse influences on the characteristics of the SAW device are produced.
In dry-etching for an Al alloy, Cl2 gas is used for increasing the etching rate of the Al alloy and also BCl3 gas having deoxidation property is added for removing a natural oxide film. Cl2 gas containing a halogen becomes a component of a protective film on the sidewalls to prevent side etching. However, when the electrode includes, besides Al, a metal whose halogenide has a low vapor pressure, the etching is finished with residue or the etching itself does not proceed. That is, even if chlorine radical is attached to the electrode, the etching does not proceed because the Al alloy has high activation energy unlike pure Al.